Different types of acoustic devices have been used through the years. One type of acoustic sensing device is a microphone. A microphone has a sensor that transforms acoustic energy into electrical signals. Microphones are deployed in various types of devices such as personal computers or cellular phones.
Capacitive microelectromechanical system (MEMS) microphones have recently become popular in consumer electronics. However, there are a number of different transduction mechanisms that can be used in microphones beyond capacitive, including piezoresistive, piezoelectric, and optical among others. What these approaches all have in common is the use of a diaphragm which displaces in response to an acoustic pressure. The transduction method then turns the displacement into an electrical signal corresponding to the acoustic pressure. One goal is to optimize sensitivity in a very small microphone which is done by maximizing the displacement of the diaphragm under acoustic pressure. This can be accomplished by using a simply supported diaphragm which is one that is free to rotate or displace at its edge. In order to rotate or displace at the edge, a simply supported diaphragm must have a gap around the majority of its perimeter. This gap however leads to acoustic leakage which in conjunction with the back volume defines the low frequency response of the microphone.
In order to create a small microphone with a simply supported diaphragm, one must control the total area of the gap at the edge of the diaphragm. The gap width may be made very small, for instance it is common to make gaps of 1 micron width. However the variation of the gap width directly affects the acoustic leak resistance and thus figures directly in the variability of the low frequency roll-off, an undesirable outcome. If the thin film(s) used to form the diaphragm have a residual stress gradient, which is very common, the diaphragm will be bowed out of plane and the gap will increase correspondingly, contributing to the response variability.
As an example, U.S. Pat. No. 6,535,460 by Loeppert describes a capacitive microphone that has a simply supported diaphragm which is free around it periphery. The gap formed around the perimeter largely determines the low frequency performance.
As a second example, U.S. Pat. No. 5,452,268 by Bernstein discloses a capacitive microphone that has a simply supported diaphragm with springs arranged around the perimeter to establish and maintain the diaphragm compliance. The gap around the majority of the perimeter largely determines the low frequency response. Residual stress gradients in the diaphragms of Loeppert '460 or Bernstein '268 add to the gap variability.
As a third example, U.S. Pat. No. 9,055,372 by Grosh describes a piezoelectric microphone that uses cantilever diaphragms, which require a gap around the majority of the perimeter. Residual stress gradient in the piezoelectric films results in the bowing of the diaphragm and larger than designed gaps along the edge.
Various types of sensors also exist that are used in microphones. In a capacitive microelectromechanical system (MEMS) sensor or device, a silicon substrate supports a diaphragm and a back plate. The MEMS sensor is disposed on a base and enclosed by a housing (e.g., a cup or cover with walls). The microphone itself includes a port that may extend through the base (for a bottom port device) or through the top of the housing (for a top port device). In any case, sound energy traverses through the port, moves the diaphragm and creates a changing potential of the back plate, which creates an electrical signal.
MEMS devices have various limitations and so piezoelectric sensing devices have been used in some circumstances. In these devices, the sound energy causes the piezoelectric materials in these devices to bend, which in turn produces an electrical charge and hence an electrical signal.
However, various problems also exist in these devices. For example, when these devices are deployed as sensors, residual stress gradients in the piezoelectric material causes it to bend, and a large gap is typically created between the back volume and front volume in the sensing device. This large gap (relative to the size of the entire device) negatively impacts device performance. Although the overall device can be made larger to mitigate the gap that is created, this dimensioning inherently makes the size of the device much larger, thereby making the device impractical to use in situations where small devices are needed or required (e.g., in small devices where space is at a premium such as cellular phones and tablets).
The problems of previous approaches have resulted in some user dissatisfaction with these previous approaches.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein.